The use of semi-permeable membranes to separate gas mixtures has become a well known technique in the production of industrial gases. Known plants for the separation of a gas mixture by such membranes are constructed so as to present a large surface area of membrane to the gas mixture to be separated. For example, such plants may employ a multitude of identical, elongate, hollow fibres which are formed of a suitable semi-permeable membrane and which extend in parallel to one another. The fibres are appropriately mounted typically in a pressure vessel. The gas mixture to be separated is fed into a port at or near one end outside the fibres. It flows longitudinally of the fibres. The insides of the fibres are maintained at a pressure lower than that which obtains on the outside of them. The components of the gas mixture diffuse through the membranes at different speeds. As the gas mixture passes along the outside of the membrane so a faster permeating compound passes more and more to the low pressure side. Accordingly, the gas on the outside of the fibres (the high pressure side) becomes richer in the slower permeating component or components as it flows along the outside of the fibres, and a product gas enriched in the more slowly diffusing component, may be withdrawn at pressure from the end of the pressure vessel opposite that at which the feed gas is introduced. The permeate gas is enriched in the faster diffusing component. The permeate gas is typically withdrawn from the inside of the fibres at the same end of the vessel as that at which the feed gas is introduced. If desired, in an alternative arrangement, the feed gas may be fed to the inside of the hollow fibres and the permeate gas withdrawn from the outside.
It is disclosed in U.S. Pat. No. 4,881,953 that the productivity of the membrane can be reduced by up to 25% by the presence of contaminants in the feed gas mixture. In, for example, the production of nitrogen from air, carry-over of oil vapour from an oil flooded air compressor or the presence of hydrocarbons or acid gases in the ambient air can have a deleterious effect on the membranes. Accordingly, measures such as the use of refrigerated air driers effective to reduce the temperature of the incoming air to below ambient temperature to remove condensible contaminants (e.g. water) and the use of activated carbon filters to treat the feed air downstream of the compressor are commonly taken to minimise the concentration of contaminants in the air entering the membrane vessels. Even if these measures are taken however, some contaminants may still reach the membrane vessel.
Conditions at the feed gas end of the membrane vessel can have a disproportionate effect on the overall performance of the gas separation apparatus. For example, a change in feed air temperature at the feed gas inlet has a large effect on product yield and purity before any change is seen in product temperature at the product gas outlet, i.e. before the membranes reach temperature equilibrium. Contamination by for example oil vapour tends also to be concentrated at the feed gas end of the membrane fibres. The combination of these effects can result in significant degradation of the performance of the apparatus before the majority of the length of the membranes becomes contaminated. It is an aim of the present invention to provide an apparatus which ameliorates the above-described problems.